A speech is compressed by a speech codec (encoder) of a transmission terminal, various types of headers such as real-time transport protocol (RTP), user datagram protocol (UDP), and Internet protocol (IP) headers are attached to the compressed speech, and the header-attached compressed speech is transmitted through a communication modem such as a long-term evolution (LTE) modem. A reception terminal sequentially removes the RTP, UDP, and IP headers and checks whether there is a lost speech frame. A speech frame restored without any loss is decompressed by a speech codec (decoder) from a compressed state, converted into a pulse coded modulation (PCM) signal, and delivered to a speaker.
When a loss in information is discovered during the restoration of speech frames, error concealment for reducing damages in sound quality is carried out by using information about previous frames. If a call is not smooth because speech frames were lost during transmission due to congestion of a transmission path, a speech bit-rate may be adjusted by transmitting a codec mode request (CMR) message to a counterpart terminal such that the bit-rate is temporarily decreased when there is a congestion status and is gradually increased as the congestion status is solved.
According to conventional voice compression techniques such as adaptive multi-rate (AMR) and adaptive multi-rate wideband (AMR-WB) techniques, a voice bandwidth is fixed to a narrowband or a wideband, this indicates that an analog voice signal is converted into a digital signal at 8,000 or 16,000 samples/s and compressed, and this conversion speed does not change during a call. An AMR codec may compress a voice signal digitized at 8,000 samples/s to eight types of bit-rates of 4.75 to 12.2 Kbps and process a voice signal of a band of 300 to 3,400 Hz. An AMR-WB codec may compress a voice signal digitized at 16,000 samples/s to nine types of bit-rates of 6.6 to 23.85 Kbps and process a voice signal of a band of 50 to 7,000 Hz.
Recently, with respect to call quality of AMR-WB voice codecs commercialized in a voice over LTE (VoLTE) service, most listeners think that call quality is improved, but some listeners do not prefer call quality of a high-frequency voice and tend to think that the quality of a conventional AMR voice codec was better. This indicates that a high-frequency voice component may or may not be preferred according to a listener or background noise compressed along with a voice.